The Bell's Vireo (Vireo bellii) is a widespread North American species of bird that has declined since the mid-1960s primarily due to habitat modification. Throughout its range, Bell's Vireo populations are regulated under varying degrees of protection; however, the species has never been characterized genetically. Therefore, the current taxonomy used to guide management decisions may misrepresent the true evolutionary history for the species. We sequenced 86 individuals for ND2 and genotyped 48 individuals for genome-wide SNPs to identify distinct lineages within Bell's Vireo. Phylogenetic analyses uncovered two distinct clades that are separated in the arid southwestern United States, near the border of the Chihuahuan and Sonoran Deserts. These clades diverged from each other approximately 1.11-2.04 mya. The timing of diversification, geographic location, and niche modeling of the east/west divergence suggest vicariance as a mode of diversification for these two lineages. Analyses of the SNP dataset provided additional resolution and indicated the Least Bell's Vireo populations are a distinct evolutionary lineage. Our genetic evidence, together with information from morphology and behavior, suggests that the Bell's Vireo complex involves two species, each containing two separate subspecies. This new information has implications for the federal, state and other listing status of Bell's Vireo throughout its range.
More than 170 subspecies are listed as threatened or endangered under the US Endangered Species Act. Most of these subspecies were described decades ago on the basis of geographical variation in morphology using relatively primitive taxonomic methods. The US Fish and Wildlife Service defaults to subspecies descriptions by taxonomists working with specific groups of organisms, but there is no single definition of subspecies across plants and animals. Valid tests today usually entail molecular analyses of variation within and among populations, although there is no reason that behavioral, ecological or molecular characters could not be used, and include tests for significant differences between samples of the putative endangered subspecies and its nearest geographic relatives. We evaluated data gathered since subspecies listed under the ESA were described finding about one-third are valid (distinct evolutionary taxa), one-third are not, and one-third have not been tested. Therefore, it should not be assumed that because a subspecies occurs in a checklist, it is taxonomically valid. If the US Fish and Wildlife Service intends to continue listing subspecies, we suggest that they convene taxonomic experts representing various groups of organisms to provide a minimal set of criteria for a subspecies to be listed under the ESA.
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